A stem cell is a cell having a self-reproducing potential and a differentiation potential. For example, an embryonic stem cell (an ES cell), an induced pluripotent stem cell (an iPS cell), and a somatic stem cell are known as stem cells. Examples of the somatic stem cell include: a hematopoietic stem cell, a neural stem cell, and a mesenchymal stem cell. Each of the ES cell and the iPS cell has a pluriopotency to differentiate into any tissue. The hematopoietic stem cell has a potential to differentiate into a blood cell. The neural stem cell has a potential to differentiate into a nerve cell. The mesenchymal stem cell is present in tissues of bone marrow or the like, and is known as a stem cell having a pluriopotency to differentiate into an adipose cell, a bone cell, a chondrocyte, or the like.
Of these stem cells, for example, the mesenchymal stem cell is currently used as a cell for transplantation in a regenerative medicine field. The mesenchymal stem cell is adapted to various diseases such as a bone defect, a cartilage defect, a periodontal disease, a myocardial infarction, a refractory dermatosis, osteoporosis, osteoarthrosis, a spinal cord injury, hematopoietic support, and antirejection in organ transplantation. It is expected that in the future, the mesenchymal stem cell will be adapted to more and more diseases (e.g., a cerebral infarction, arteriosclerosis obliterans, a kidney disorder, etc.).
Currently, a culture medium to which an animal serum (generally 10 to 15% fetal bovine serum) is added is extensively used as a culture medium for inducing differentiation of the stem cells into various functional cells. Such a serum is used as a nutrient source for promoting in vitro cell growth and/or proliferation, or a supply source of a biologically active substance such as hormone.
However, a serum is very expensive, and components thereof differ for each lot because the serum is a natural product. This is highly likely to cause variation in promotion effect of cell differentiation. Moreover, cultured cells need to be purified so that serum-derived proteins or the like are removed from the cultured cells. This requires a complicated process. Furthermore, there is a risk that cultured cells may be infected with an unknown pathogen (such as a virus or pathogenic prion) that is included in the serum. Further, for example, in case of inducing differentiation of a mesenchymal stem cell by use of a culture medium containing a serum, the serum may contain a component inhibiting bone differentiation of the mesenchymal stem cell. This may cause bone differentiation of the mesenchymal stem cell to be carried out with lower efficiency. In view of the circumstances, a technique which allows inducing differentiation of the above-mentioned stem cells under a serum-free condition is essential to realization and spread of regenerative medicine.
For example, in order to induce differentiation of a mesenchymal stem cell into an osteoblast cell, a method is known for carrying out bone differentiation by use of a reduced amount of serum. For example, a method is disclosed for inducing differentiation of a mesenchymal stem cell into an osteoplastic cell by adding an epidermal growth factor (EGF) to a culture medium in which an amount of fetal bovine serum is reduced to 1% (see Non Patent Literature 1).
Further, a technique is also known in which a composition containing a specific growth factor and a phospholipid is added to a basal medium, so as to induce a cell, which has been obtained by culturing a mesenchymal stem cell under a serum-free condition, to differentiate into an osteoblast cell or an adipose cell with the use of a serum-containing culture medium (see Patent Literature 1).